A device is known for hydrodynamic needling of fleeces, tissues, or paper with a liquid processing means sprayed from a plurality of nozzles against the material, said device consisting of a sheet metal drum associated with the nozzles and possibly subjected to an internal vacuum, said drum serving as a supporting element for the material, said means additionally being covered at its circumference by a covering that is permeable to liquid, with a support being provided between the liquid-permeable covering and the sheet metal drum to increase the distance between the drum and the covering.
A device of this kind is also known from DE-GM 1 886 883 for through-flow heat treatment of textiles. In this document, a screen fabric with a coarser wire diameter is proposed as the support. This additional drum covering has the advantage that the material resting on the perforated drum can be ventilated more uniformly than if the material is in direct contact with the perforated sheet metal drum. The material to be treated, because of the additional screen fabric, is located at a greater distance from the jacket surface of the drum so that no dead spots, i.e. areas through which there is no flow, do not occur on the surface of the material.
Another screen drum design is known from DR 39 05 736 A1. In this design, a perforated sheet is not used to make the drum, but sheet metal strips extending in the axial direction run instead between the two bottoms of the drum, with spacers located between the strips sheet metal strips and being held together by screws. This sheet-metal-strip spacer design makes the drum stable without using the sheet metal of the drum as a screen. It is therefore optimally permeable to air, but costly to manufacture.
Finally, reference can be made to DE 44 22 508 C1 according to which sheet metal strips that extend axially in a straight line for the entire length of the drum and serve as a support are distributed in multiples around the circumference of the drum. The strips are made rectangular or round and must be welded to the drum. This fastening, however, causes the sheet metal of the drum to warp. In addition, the covering in that document consists of a screen fabric in which individual fibers can become trapped, resulting in contamination that is difficult to remove.
In the field of water needling, in other words hydrodynamic interlacing of the fibers of a material such as tissue or the like that is moved beneath the streams from the jets to compact it, U.S. Pat. No. 3,485,706 could be cited. Basically, the use of finely perforated sheet metal as a coating on a permeable drum is known from this patent. With such smooth, finely perforated sheet metal that replaces fabric made of woven wires, smoother compacted fleeces are produced by water needling, since the smooth sheet metal has a sort of ironing effect while nevertheless serving because of its permeability to carry away the water sprayed on it. However, it is difficult to control clogging of the fine holes in the sheet metal that carry away the liquid, said holes readily becoming clogged with fibers of the fleece or with deposits of the liquid that is sprayed. In this connection, EP 0 223 614 B2 could be mentioned. Here again, the covering consists of finely perforated cylindrical sheet metal. The drum design that supports the sheet metal in that patent consists of a perforated cylinder with ribs aligned axially lengthwise that are located between the rows of holes and project radially, each tapering to form a point. The manufacture of such a drum is very costly and tedious.